The present invention relates to an equilin hapten, to an immunogen for preparing an antiserum especially suited for use in the radioimmunoassay of equilin and to the antiserum. The invention also relates to a method of radioimmunoassay using the antiserum.
Equilin, in the form of its 3-sulfate salt, is a major component of conjugated (equine) estrogens which are used for the treatment of menopausal disorders. The three major components of conjugated (equine) estrogens by weight are estrone sulfate (about 50%), equilin sulfate (about 25%) and 17.alpha.-dihydroequilin 3-sulfate (about 15%). Although equilin is known to be a potent estrogen, little is known about its pharmacologic action and metabolism. A tool, which would be extremely useful to acquire knowledge about these parameters, would be a specific antiserum for equilin.
Specific antisera have been developed for such estrogens as estrone, 17.beta.-estradiol and estriol, for example, see P. N. Rao and P. H. Moore, Jr., Steroids, 29, 461 (1977). An antiserum also has been reported for equilin, see P. G. Whittaker et al., The Lancet, 1, 14 (1980) and M. R. A. Morgan et at., J. Steroid Biochem., 13, 551 (1980); this antiserum is reported to have a cross reaction (4%) with the two major components of conjugated estrogens. A characteristic which distinguishes the latter antiserum from the present antiserum is that it is derived from an immunogen obtained by attaching a steroid to an immunological carrier by means of an ester linkage, a form of attachment different from that of the present immunogen. Notwithstanding these developments, a specific antiserum equilin would be of great value for determining equilin levels in plasma.
Accordingly, the present invention fulfills the above-noted need by providing a specific antiserum for the radioimmunoassay of equilin. The antiserum has a minimum of cross reaction with other steroids found in serum and/or steroids which form the conjugates present in conjugated estrogens.